Programming Tutorials

Finding slow queries in MySQL - Enable slow query log.

By: Murali in MySQL Tutorials on 2023-05-17  

In MySQL, you can check which queries took longer to execute and potentially caused the server to slow down by enabling the Slow Query Log feature. The Slow Query Log records queries that take longer than a specified amount of time to execute. Here are the steps to enable and utilize the Slow Query Log:

1. Open the MySQL configuration file, typically located at /etc/mysql/my.cnf or /etc/my.cnf, depending on your system.
2. Look for the [mysqld] section in the configuration file.
3. Add or modify the following lines to enable the Slow Query Log

   slow_query_log = 1
   slow_query_log_file = /path/to/slow-query.log
   long_query_time = 2
   

   • slow_query_log = 1 enables the Slow Query Log.
   • slow_query_log_file specifies the file path where the log will be saved. Replace /path/to/slow-query.log with the desired location.
   • long_query_time sets the threshold time in seconds. Queries taking longer than this value will be logged. Adjust the value as per your requirements.
4. Save the configuration file and restart the MySQL server for the changes to take effect.

After enabling the Slow Query Log, MySQL will start logging queries that exceed the specified execution time to the specified log file.

To analyze the log and identify slow queries, you can follow these steps:

1. Open the slow query log file specified in the configuration.
2. Look for queries with a duration exceeding the long_query_time value.
3. Analyze the queries to identify potential performance bottlenecks, such as missing indexes, inefficient queries, or suboptimal database design.
4. Optimize the identified queries or make necessary adjustments to improve performance.

There are also tools available that can help analyze the Slow Query Log and provide insights into query performance, such as mysqldumpslow or third-party monitoring tools like Percona Toolkit or pt-query-digest.

Keep in mind that enabling the Slow Query Log can have an impact on server performance and disk space usage, so it's recommended to enable it for specific troubleshooting or performance analysis purposes and disable it when not needed.